In general, hearing aids are hearing devices to assist hard-of-hearing persons who have a weak hearing capability to hear better external sound. However, since the hearing aids are so small compact devices that users can wear the hearing aids in their ears or behind their ears in the normal use, the hearing aids will be programmed individually by experts for fitting hearing aids according to prescription in order to amplify the frequency range that it is difficult for users to recognize before use of the hearing aids. Here, as shown in FIG. 1, even if a signal is amplified by a digital amplifier in a digital signal processor (DSP) integrated circuit (IC) chip (D), the digital hearing aid may change an operational status of the digital amplifier via a volume controller (VC), in order for a user to adjust the degree of amplification according to a user hearing capability. Here, most of the volume controllers (VC) are variable resistor type volume controllers, or memory change digital button switches. However, according to the problem of the above-mentioned digital hearing aid, when the hearing aid is inserted and worn in the ear, there is a gap between the outer surface of the hearing aid and the surface of the skin of the ear canal, a sound output from a hearing aid receiver (R) is not delivered to the eardrum only, but leaks through the gap, to then be input to a hearing aid microphone (M) again and to thereby cause an acoustic feedback noise problem (F). The acoustic feedback noise is usually a “beep” sound, to cause very severe discomfort to the hearing aid user and to disable the hearing aid user to boost and hear others' voice. In addition, if a user who uses the hearing aid says or chews food at a state of having worn the hearing aid, the internal diameter of the ear canal becomes wider due to the structural properties of the ear canal, and thus feedback noise cannot but occur. Therefore, the above-mentioned conventional digital hearing aids require measures of removing the feedback noise.
Here, as the conventional technology related to the hearing aids, the Korean Patent Laid-open Publication No. 10-2001-0008008 on Feb. 5, 2001 entitled Automatic fitting method of Hearing aids was proposed by the applicant Ms. Yoonjoo, Sim.
Referring to FIG. 2, a conventional hearing aid having an acoustic feedback function, includes an analog-to-digital (A/D) converter 71 that converts an analog input signal tone input from a microphone (M) 70 into a digital signal to output the conversion result;
an input buffer memory 72 that stores the digital input signal tone data output from the A/D converter 71, to then sequentially output the stored digital input signal tone data;
a subtracter 74 that subtracts a y scalar signal (y) of a feedback buffer memory 73 for eliminating a feedback signal, from input signal tone data (d) output from the input buffer memory 72;
an intermediate buffer memory 75 that stores the input signal tone data (e) output from the subtracter 74, to then sequentially output the stored input signal tone data;
an amplifier 76 that amplifies the input signal tone data output from the intermediate buffer memory 75, to then output the amplified result;
an output buffer memory 77 that stores the input signal tone data output from the amplifier 76, to then sequentially output the stored input signal tone data;
a digital-to-analog (D/A) converter 79 that converts the digital input signal tone data from which feedback noise has been removed and that is output from the output buffer memory 77, into an analog signal, to then output the analog signal to a receiver 78;
a feedback output buffer memory 80 that makes part of the amplified signal output from the output buffer memory 77 feedback to then be stored as x vector data;
a coefficient updating unit 81 that updates the coefficient of the input signal tone data output from the intermediate buffer memory 75;
a counter 82 that stores N coefficients of the updated input signal tone data output from the coefficient updating unit 81 as w vector, to then output the stored w vector; and
a multiplier 83 that multiplies x vector data of the feedback data output from the feedback output buffer memory 80, by the w vector data of the counter 82, to then output a y scalar value to the feedback buffer memory 73.
On the other hand, the conventional hearing aid having the acoustic feedback function operates that the A/D converter 71 converts an analog input tone signal input from a microphone (M) 70 into a digital signal to then output the conversion result to the input buffer memory 72. In addition, the input buffer memory 72 stores the digital input signal tone data output from the A/D converter 71, to then sequentially output the stored digital input signal tone data to the subtracter 74. In this case, the subtracter 74 subtracts a y scalar signal (y) of the feedback buffer memory 73 for eliminating a feedback signal, from input signal tone data (d) output from the input buffer memory 72, to then output the resultant signal (e=d−y) to the intermediate buffer memory 75. The intermediate buffer memory 75 stores the input signal tone data (e) output from the subtracter 74, to then sequentially output the stored input signal tone data to the amplifier 76 and the coefficient updating unit 81. In addition, the amplifier 76 amplifies the input signal tone data output from the intermediate buffer memory 75, to then output the amplified result to the output buffer memory 77. In addition, the output buffer memory 77 stores the input signal tone data output from the amplifier 76, to then sequentially output the stored input signal tone data to the digital-to-analog (D/A) converter 79 and the feedback output buffer memory 80.
In this process, the feedback output buffer memory 80 temporarily stores the signal amplified by the amplifier 76 prior to being output to the receiver 78 through the D/A converter 79. Here, the data stored in the feedback output buffer memory 80 is stored as the x vector of not single digital data but N data. According to a storing sequence of the feedback output buffer memory 80, the oldest data is in the first stage of the feedback output buffer memory 80, the latest data is located at the end of the feedback output buffer memory 80, the oldest data is deleted every sampling time, and then the next oldest data is transferred to the first stage. The counter 82 stores N coefficients output from the coefficient updating unit 81 as w vector, to then output the stored w vector to the multiplier 83. The x vector data of the feedback output buffer memory 80 is also output to the multiplier 83. Accordingly, the multiplier 83 multiplies x vector data output from the feedback output buffer memory 80, by the w vector data of the counter 82, to then output a y scalar value to the feedback buffer memory 73.
In this process, if the output tone output from the receiver 78 is fed back and then input back to the microphone 70 to thus cause acoustic feedback, the feedback noise is amplified again to thus cause further amplified feedback noise and to thereby repeat a vicious cycle of feedback noise.
Thus, in order to reduce or eliminate the feedback noise, the previously calculated y scalar (y) is subtracted from the feedback input tone (d), and the subtracting result (e=d−y) is input to the amplifier 76. Here, the subtracting result (e) that is obtained by subtracting the y scalar (y) from the input tone (d) is removed by updating the w vector every sampling time in the coefficient updating unit 81 from the x vector of feedback output buffer memory 80 and the w vector of the counter 82, and then moving the resultant N updated w vector data to the counter 82.
However, the conventional hearing aid having the acoustic feedback function inevitably needs 2N+1 multiplications and additions among the entire operation to eliminate the feedback noise, because at least 128 integers are used as a value of N in most cases. As a result, a problem of causing a large load to the hearing aid system occurs due to the entire operations. In addition, certain ambient noise due to an acoustic feedback signal and a narrow frequency band is not normally removed, to thus cause discomforts to the hearing aid user due to the acoustic feedback noise as well as cause a problem of significantly lowering speech discrimination.